1. Field of the Invention
The present invention relates to an apparatus and method for stabilizing the heating and cooling of a dome in an inductively coupled RF plasma reactor of the type having a chamber dome overlying a semiconductor substrate being processed.
2. Description of the Prior Art
Control of the temperature of process chambers that are used to etch, implant, or deposit material on semiconductor substrates, is necessary to provide reliable and reproducible semiconductor fabrication processes. Many of these processes are highly temperature dependent and provide different processing rates or characteristics at different temperatures. Temperature fluctuations of a chamber are particularly undesirable during sequential processing of a batch of substrates, because the substrates are processed with different properties. For example, in etching processes, temperature fluctuations can cause the shape of the etched features on the substrate to vary widely from one substrate to another, and to vary as a function of the temperature profile across the substrate. Also, large temperature fluctuations of the chamber components or walls can cause residues that deposit on the chamber walls or ceiling to flake off and contaminate the substrate.
Another temperature control problem arises when the chamber walls and surfaces are made of ceramic materials, since ceramic materials often have a low thermal shock resistance and crack when subjected to thermal stresses resulting from large variations in temperature. For example, a chamber wall is made from a ceramic material, such as aluminum oxide, which has a low tolerance to thermal stress, when inductor coils are used to couple RF energy into the chamber. Ceramic materials that have high thermal expansion coefficients undergo a large expansion or contraction for even a small temperature change causing both the deposition and the wall to break or crack when subjected to widely different temperatures. It is desirable to control the temperature of the ceramic surfaces of process chambers and to reduce their temperature fluctuations.
Conventional temperature control systems for semiconductor process chambers include “water-jacketed” liquid recirculator systems or forced-air cooling systems. In certain processes RF induction energy is used a means to heat the process chamber. When RF is used, it irradiates the chamber and the included work piece materials through a dome at the top of the chamber. The inductor antenna is typically adjacent to the dome, which is transparent to RF energy, and essentially inductively couples RF energy into the chamber to form high-density plasma therein. Because circulating cooling liquids absorb RF forced, air-cooling is a frequent choice when this type of heat is utilized. However, forced air particularly may results in instabilities and localized hot spots on the dome. The invention herein is directed to forced-air cooling systems that are used in maintaining dome temperatures.
Forced-air cooling systems, as for example, described in U.S. Pat. No. 5,160,545, issued Nov. 3, 1992, use fans to blow cooled air past chamber surfaces. These systems often cause localized hot spots at portions of the chamber that are shielded from the air flow. Also, because the primary mode of heat transfer is conduction by air, forced air cooling
Typically requires a large airflow to achieve acceptable response times to large temperature fluctuations in the chamber, such as the temperature fluctuations caused by turning on and off the plasma. The large airflow rates also typically require large fans, which are switched on and off, resulting in low reliability of the cooling system. Such breakdowns often result in damage chamber components.
It is desirable to have a process chamber temperature control system capable of producing stable temperatures to overall reduce large temperature variations in process chambers. It is further desirable for the temperature control system to stabilize temperatures in a manner that does not interfere with the operation of work pieces in the chamber, or the chamber components such as the coupled RF energy. It is further desirable for the temperature control system to reduce or eliminate thermal stresses on the chamber dome surface.